Show your own screenshare track and offer tab audio in the picker

[MinitorMHS]

Two small screenshare fixes. They're unrelated in code but both affect the
screenshare path, so I'm putting them together to save a review round-trip —
happy to split if you prefer.

1. Your own screenshare tile renders empty

Problem

Cloudflare Calls doesn't loop a published track back to its sender, so
pulledVideoTrack is always undefined for the self participant. Participant
already special-cases self for the camera, but the screenshare case fell
through to the pulled track and rendered nothing.

Change

Extend the self special-case to cover screenshares, using the local capture
track:

const videoTrack =
  isSelf && !isScreenShare
    ? userMedia.videoStreamTrack
    : isSelf && isScreenShare
      ? userMedia.screenShareVideoTrack
      : pulledVideoTrack

screenShareVideoTrack is an useObservableAsValue of
screenshare.video.broadcastTrack$, mirroring the existing camera case.

2. No way to share tab audio

Problem

getScreenshare({ audio: false }) calls getDisplayMedia without an audio
constraint, so Chrome and Edge don't render the "Share tab audio" checkbox.

Change

Pass audio: true to getScreenshare.

Notes on E2EE and stream counts

Neither change adds a stream to the room or affects MLS.

• The tile fix uses the same unencrypted local capture the camera path already
  uses for self — it's what you captured, not a decrypted remote stream.
• The picker change only affects what the picker offers. The captured audio
  track isn't published — nothing subscribes to the screenshare audio
  broadcast track — so sender stream counts are unchanged.

Actually publishing screenshare audio would be a separate change: it adds an
MLStream per sender, and the MLS group has to be reconfigured when a sender's
stream count changes.